The known multistage processes for producing and working up cellulose ethers have been comprehensively described in R. DÖNGES, “Entwicklungen in der Herstellung and Anwendung von Cellulosethern” [Developments in the production and use of cellulose ethers], “DAS PAPIER”, 12/1997, pp. 653-660. This publication describes processes for carrying out the alkaline activation of the cellulose and respective etherifications thereof, and the subsequent workup and purification procedures of the resultant cellulose ethers for removing salts, minor organic components and the regeneration of suspension media.
These processes are disadvantaged in that they require a multiplicity of large usually pressure-tight vessels having mechanical agitators and mixers, especially heat exchangers, various transport, separation and drying and comminution units, and comprehensive control and instrumentation systems. The processes are further disadvantaged in that they are time-consuming and demanding in terms of equipment and are extremely material- and energy-consuming and produce polluted liquid waste products.
The known processes for preparing cellulose esters may also be characterized in a comparable manner. Illustratively, the production and workup of cellulose nitrate (usually called nitrocellulose) have been described in numerous publications, for example German Auslegungsschrift 1 771 006, U.S. Pat. Nos. 4,401,809, 1,818,733, 1,911,203, German Offenlegungsschrift 2 051 210. According to these processes, high-volume stirred tanks, stirred autoclaves, centrifuges and various other capital-intensive machines and apparatus are used, not only in cellulose nitration, but also for deacidifying the nitrocellulose and especially for standardizing product specifications, for example solution viscosity and chemical stability. This, is still the state of the art and characteristic of said production processes.
The batchwise procedure, which still prevails today in most fabrication stages of the known processes for producing and/or working up polysaccharide derivatives, is characterized by considerable complexity and personnel requirements. Moreover, the procedure causes variations in processing and quality parameters, as a result of which subsequent compensation measures (for example complex remixing operations of individual batches in correspondingly large mixing apparatuses) for post-standardizing the desired product specifications are the norm.
It is, therefore, an object underlying the present invention to provide a process which is continuous throughout for the preparation and/or workup of polysaccharide derivatives, for example cellulose derivatives or starch derivatives.